Finally completed my PPF delete w/ the KMiata rear diff brace and a V8R trans support. Yay. Now that the PPF is gone i've gotta figure out where to run this ground that used to connect to the PPF. Since i'm an electrical dunce I need some ideas where to run this ground to since the Miata already seems to have a so-so reputation for bad grounds.

You might consider grounding it at the trans somehow. This ground goes from the battery, bolts to the chassis right behind it, then goes down through a grommet where it’s grounded at the ppf. So going to the body or subframe is already taken care of in the trunk. The one on the ppf helps ground the engine along with the strap behind the header

I am not an EE, but I drilled my PPF delete for a rivnut and stuck the ground there. The car did not seem to mind that.

I am.

The PPF served an interesting function in this regard. It electrically tied the front and rear of the drivetrain together. The ground strap which originally went to it actually is not 100% necessary, as the battery is locally tied to the body in the rear, and the engine (including the alternator) is tied to the body at the front, on the exhaust side.

That being said, redundancy is good. If you want to duplicate this redundancy, do what curly suggests and run a new ground wire from battery (-) up to some point on the transmission.

Thank you.

When I did my ppf delete for the BMW transmission I neglected to run the additional ground from the ppf to the transmission and suffered a melted wiring harness on both cooling fan circuits and a few other interesting places. I would highly recommend doing as Joe suggested and running a large gauge ground to the transmission or engine block.

Yeah do what Joe said which was listen to curly which was ground to the trans!

If you are going to run cable, which you should. Terminate it at the engine for a better result than to the trans, not much extra cable to do that. The miata rear chassis ground is depending on welded old sheet metal at the rear of the car and that's why mazda had the ground cable terminate in two points.

Why? The electrical resistance of the transmission case, and at the union between the transmission and the engine block, has gotta be so low as to be immeasurable with conventional instruments.

The additional length of wire to reach the engine would add far more resistance to the path than the transmission case.

The transmission case is saturated with oils, bolted together with sealers, is not copper, has many seams that oils can find their way into. I'd highly doubt the resistance would be less. But to each their own. I would opt for a direct path with an important main ground. Also bolting to the transmission case where? One would have to drill and tap the case on the webbing or use the large 12/14mm ppf mounting holes and that would mean the cable is going to be suspended in the air from trans tunnel to trans.